A tower \( PQ \) stands on a horizontal ground with base \( Q \) on the ground. The point \( R \) divides the tower in two parts such that \( QR = 15 \, \text{m} \). If from a point \( A \) on the ground the angle of elevation of \( R \) is \( 60^\circ \) and the part \( PR \) of the tower subtends an angle of \( 15^\circ \) at \( A \), then the height of the tower is :